In most ethics trainings I have taken we are told to avoid taking actions that could lead to the perception of impropriety because the perception of impropriety can be as bad or worse than impropriety itself.

That ethos becomes more important the higher ones station becomes in politics and business.

I am not accusing Yellen of anything improper, however these transactions give circumstantial support to those who may believe she acted or will act in an improper way.